Thermal processing - Calculate D value

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Calculate D value

Printed From: Dairy Science and Food Technology
Category: Thermal processing
Forum Name: Thermal processing
Forum Description: Effects of heat on food, enzymes and microorganisms
URL: https://www.dairyscience.info/forum/forum_posts.asp?TID=316
Printed Date: 18 Apr 2024 at 7:05pm
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Topic: Calculate D value

Posted By: mstrange
Subject: Calculate D value
Date Posted: 11 Feb 2012 at 5:09pm

Hi again!

Got table of time (mins) and nos of survivors at 200F. Pl can you tell how to calculate D at 200F?

Thanks.

Replies:

Posted By: formula123
Date Posted: 11 Feb 2012 at 11:37pm

Welcome back.

I am going to explain how to do this by calculating the slope of the thermal death curve for the organism you have studied using an Excel function called "slope". The D-value is then obtained by obtaining the negative reciprocal of the slope.

1. I presume that you have these values in an Excel spreadsheet? If not create a workbook with 3 columns, Time (minutes), Colony forming units/ml, and Log₁₀colony forming units/ml.

2. Convert your colony forming units (CFU)/ml values to their log₁₀equivalents.

3. Excel 2010 has a useful slope function to calculate the slope of the linear regression line for x and y values and I will explain how to use this function.

4. Lets assume that your time values (x values) are in column A and range from A2 to say A15 and the corresponding log₁₀colony forming units/ml are in column C and range from C2 to C15. The slope can then be calculated using =slope(C2:C15, A2:A15).

5. The D-value is obtained by obtaining the reciprocal of the slope and multpling this value by -1.

6. I suggest that you produce a figure showing the relationship between time and the reduction in Log₁₀CFU/ml. Show the experimental values and add a trendline. Use Excel to calculate the linear regression equation and show the r²value. If this is less than 0.9 consider your error sources and look for "tailing" of your data points.